Rigidbody.AddRelativeForce should be adding force over time, and giving you that smooth acceleration that you're looking for. However, I notice you have it set incredibly high, so it may simply be that you're adding so much force that it looks like its starting instantly.
Try lowering the force value from -500000. Also, if that doesn't work, there is a variable called rigidbody.drag. It slows down the speed of the rigidbody as it moves. You could try increasing the drag of your plane by setting the drag to be higher.
One last thing to look at is the mass. AddForce and AddRelativeForce both take the mass into account when looking at adding forces. Maybe you have your mass set too low, and the force just moves it too fast.
Fiddling with those three things should fix your problem.
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